Page 74 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Polymer Structure (Morphology) 37

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FIGURE 2.12 End-to-end distances for four 30-unit chains.

FIGURE 2.13 Helical conformation of isotactic vinyl polymers. From Gaylord, N. (1959): Linear and
Steroregular Addition Polymers, N. Gaylord and H. Mark, eds., Wiley, NY.

bonding, occurs in both structures. With sheets, bonding occurs between chains and for helical
structures bonding occurs within the same chain. Further, some compounds, such as α-keratin,
form helical structures within sheet structures.
α-Keratin (composed of parallel polypeptide α-helices) and most globular proteins are char-
acterized by intramolecular bonds. More about the structures of the two types of keratins is dis-
cussed in Section 10.2. These, and many other polymers, including nucleic acids, form helices.
Ribonucleic acid (RNA) exists as a single-stranded helix, while DNA exists as a double-stranded
helix. For both natural and synthetic polymers, these helices vary with respect to the number of
backbone carbons per complete cycle. Figure 2.13 contains helical conformations for isotactic
vinyl polymers. The “R” groups are designated by the larger open circles. From left to right the

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